Understanding CAN Data Protocols and Mechanisms

Non-Return-to-Zero (NRZ) method

In industrial applications, the CAN protocol allows various industrial system components, such as machines, sensors, and actuators, to communicate with one another. Data is converted to digital signals (0 and 1), with 0 called “dominant” and 1 called “recessive.” If both are transmitted simultaneously, the dominant takes priority. CAN also uses the Non-Return-to-Zero (NRZ) method for data conversion. Its overarching goal is to increase the safety and quality of automobiles, industrial machinery, and automation systems.

Especially for engineers who work with high-level industrial embedded systems, time, efficiency, and proper communication of devices are crucial. CAN protocols offer significant benefits to these professionals.

• Lower cost by driving down the need for complex wiring systems.
• Built-in error detection system that can spot errors and prevent faults from spreading.
• A robust communication protocol that can recover from errors on its own.
• Flexibility due to every device on the network having a separate CAN controller chip. This makes modification and corrections quick and easy with minimal overall system impact.

• Used by car manufacturers for aftermarket vehicle and fleet tracking.
• Vehicle remote start and security applications.

• Connecting elevator and escalator systems.
• Data transfer between access control nodes and secure door controllers.
• Connecting light control sensors and dimmers.

The format for data transmission is called the “data frame.” There are two types of data frames: standard format and extended format.

The values indicated here show the maximum bit length of each field. The upper line indicates recessive and the lower line indicates dominant. A field is fixed as dominant if a line is only shown for dominant. Similarly, a field is fixed as recessive if a line is only shown as recessive. If lines are shown for both dominant and recessive, it can vary depending on the transmitted data.

• SOF: The part to be transmitted first from the relevant note. This field is called “start of frame”, indicating the start position and is used for synchronization between nodes.
• ID: This field consists of 11 bits in standard format, 29 bits in extended. Determines data content, transmitter ID, and arbitration priority.
• RTR: The remote transmission request (RTR) field, which distinguishes between data frames and remote frames, is dominant for a data frame and recessive for a remote frame. As with the ID, this field is also used for arbitration.
• Control Field: Consists of the identifier extension (IDE) bit, the reserved bit “r” (1 bit each), and the 4-bit data length code (DLC).
• Data Field: The body of transmitted data is called the “data field.” The length varies between 0 and 8 bytes (0 to 64 bits) depending on the DLC setting, and the designer can determine what data is assigned.
• CRC Sequence: The transmitting node calculates the values to transmit for the SOF, ID, control field, and data field, and it transmits the results as the cyclic redundancy check (CRC) sequence. The receiving node also conducts the same calculation to determine if it has received the data properly.
• CRC Delimiter: Indicates the end of the CRC sequence. The combination of CRC sequence and CRC delimiter is called the CRC field.
• ACK Slot: Verifies if the data has been received correctly.
• ACK Delimiter: Indicates the end of the ACK slot. The combination of the ACK slot and ACK delimiter is called the ACK field.
• EOF: The end of frame (EOF) field is transmitted at the end of a data frame.
• ITM: Intermission, indicating bus idle state, not part of the data frame.

• Base ID: Equivalent to the standard format ID.
• SRR: 1-bit recessive substitute remote request.
• IDE: 1-bit recessive identifier extension.
• Extended ID: Combines 11-bit standard ID with 18-bit extended ID, providing ~5.4 million identifiable types.

In CAN communication, a node that will request certain data transmits a remote frame, and then the node that has the data returns a data frame. A remote frame is structured by deleting a data field from a data frame (or, by setting the DLC and data field to 0 bytes). The RTR is set to recessive to identify a remote frame (it is set to dominant in a data frame). However, to reduce the data occupancy ratio, remote frames have hardly been used in recent years. Instead, data frames are regularly transmitted to each node, in general.

In the days after CAN came into existence, the processing capacity of microcontrollers and CAN controllers was low, so the “overload frame” was developed to address this problem. The main role of this frame is to be transmitted to a node that is slow in processing a data frame, thereby delaying the start of transmission of the next data frame.

This frame is transmitted when a communication error (among the various types of communication errors) occurs, thereby indicating this occurrence on the CAN network.

The relationship between CAN and the OSI model is that CAN is a network type that focuses only on the last two layers — Physical and Data Link. The Physical layer defines how wires, connectors, and other physical connections are made. The Data Link aspect focuses on how data is packaged and sent over a network.

• The accelerometer uses its sensor to measure acceleration and generate an analog signal.
• The CAN node converts the generated analog signal to a digital signal.
• The CAN node encodes and sends the digital signal into a CAN frame format.
• The CAN node transmits the frame using the transceiver.
• Other CAN nodes receive the frame. They can then use the acceleration data for monitoring, logging, or other purposes.

KEYENCE offers a line of measurement DAQ devices or data loggers that support CAN communications. These DAQ Acquisition systems can acquire data, such as temperature, pressure, and flow rate, from CAN devices.

Also referred to as ISO 11898-2 or CAN C, High-speed CAN supports a transfer rate of up to 1 Mbit/s and is implemented with two wires. Some high-speed CAN devices are engine control modules, antilock brake systems, and emissions systems.

Also called CAN B or ISO 11898-3, low-speed CAN networks support a transfer rate of up to 125 kbit/s and are implemented with two wires as well. Typical low-speed CAN applications are wires that pass through a vehicle’s door, seat control modules, and other comfort devices in automobiles.

KEYENCE offers a line of DAQ systems that can be used in different industrial and automation applications. These portable data acquisition systems can accurately measure all types of data input, providing real-time measurements and inspections.

Have more questions about DAQ hardware solutions? Contact us today.